The studies of McMaster et aS. in 1936 (1, 2) provided the first experimental evidence that the lymphatic system might be a site of formation of antibodies, in contrast to the view generally held at the time that the reticuloendothelial system formed these substances (3). Ehrich and Harris (4) confirmed and amplified these findings, including studies of the cell content and antibodies in the lymph entering and emerging from the sole lymph node draining the site of injection of antigen in the rabbit. Again, Dougherty, Chase, and White (5) minced pooled lymph nodes from mice injected with antigens, and prepared a cell mass rich in lymphocytes from which they extracted antibodies to the antigen used. Harris et al. (6) gave further evidence of the association of the lymphocyte with the production of antibodies by finding these substances in higher concentration in lymphocytes collected from lymph as it emerged from a lymph node draining the site of injection of antigen.Other cells of mesodermal origin have, however, been regarded as possible sources of antibodies. Thus, the macrophages of the reticuloendothelial system were proposed in consequence of the work of Sabin referred to above (3). The plasma cell has been suggested as having such a function by Scandinavian workers. Bj~rneboe and Gormsen observed proliferation of plasma cells in the spleen and other organs of rabbits which had received repeated injections of massive doses of bacterial cells (7), and extracted antibodies from some of these sites (8). Fagraeus (9) explanted red and white splenic pulp from rabbits similarly treated, and found more antibody in explants of white pulp, which she observed to be relatively richer in plasma cells.The recent applications of histochemistry to physiology have offered an approach to the problem of the direct identification of cells concerned with the production of antibodies. It was first shown by Caspersson (10, 11) and Brachet (12) that cells which were actively forming new protein were characterized by large amounts of ribonucleic acid in their cytoplasm. Additional evidence for the association of cytoplasmic ribonucleoprotein with protein synthesis has been reviewed